This invention relates to the art of heart pacing and has particular relationship to heart pacing which is adapted to the physical activity of the host in whom a heart pacer is implanted. During physical activity the muscles absorb oxygen and glucose from the blood, generating carbon dioxide and producing other physiological and chemical changes in the blood. The pumping of the heart circulates the blood which supplies the oxygen and glucose, and the absorption of these components during physical activity is compensated in a person whose heart is operating normally by pumping of the heart at a higher rate and with an increase in stroke volume, thus pumping a greater volume flow rate of blood. The blood flow may increase by a multiple of between 3 and 5 as a result of increase in heart rate and only by a multiple of 1.3 to 1.5 in stroke volume as a result of increase. The increase in heart rate is therefore the predominant factor in the compensation. A pacemaker that directly senses muscular response during physical activity and increases the heart-beat rate accordingly would greatly increase the capacity of the host of the pacemaker for physical activity. It is an object of this invention to provide such a pacemaker.
The article entitled, Research Leads to Major Breakthrough in Rate Responsive Pacemaking, by Kenneth M. Anderson on pages 89 through 93 of Medical Electronics of Oct. 1986, describes a number of attempts at providing pacemaking whose rate is varied responsive to physical activity and lists some of their drawbacks. Kresh--Facing Sensors for Heart- and Biophysical Telemetry--35th ACBM Conference, and Kresh et al.--Closed Loop Control of Heart Rate Basic Considerations--AAIM 17th annual meeting, May 9-12, 1982, are also of interest. Anderson also describes in his paper briefly what he calls the "Activitrax" pacemaker which is described in more detail in his U.S. Pat. No. 4,428,378. An earlier pacemaker of the same type is described in Dahl, U.S. Pat. No. 4,140,132. Both of these pacemakers are alike in that in each case the body acceleration produced by the physical activity is imparted to a mass and converted into an electrical signal by a piezoelectric crystal. In Anderson the mass is the casing of the pacemaker; in Dahl, a lead block. In Anderson and Dahl, the pacer rate is not directly dependent upon the muscular response to physical activity. Both suffer from the disadvantage that what is essentially their accelerometers respond not only to the physical activity of the host in whom the pacemaker is implanted, but, also, to the acceleration and physical shocks to which the host may be subject overall, for example, in a vehicle moving on a rough or bumpy road or in driving a truck or a tractor or in any other vibrating structural body. Notwithstanding the statement in the right-hand column on page 92 of the Anderson article, the electrical signals produced by vigorous vibration of the host, which do not demand an increase in the pacing rate, may be substantial. The Anderson and Dahl pacemakers will also fail to respond to activity in situations in which an accelerometer is not responsive, as when the host is swimming.
In Gonzalez U.S. Pat. No. 4,201,219 the frequency of the pacemaker is modified by electrical signals detected in the nerve system relating to the control of the heart, i.e., to the cardiac contractions and lungs. Gonzalez's pacemaker is complicated by the supply of the neurosignals through a separate electrode in the pacemaker. In Krasner U.S. Pat. No. 3,593,718 the pulse rate of the pacemaker is varied in response to a physiological function such as the breathing rate. In this case also, the signal is supplied through a separate electrode.
It is an object of this invention to overcome the disadvantages and drawbacks of the prior art and to provide a method of variable-rate pacemaking in whose practice the rate shall be dependent on substantially any and all physical activity of the host in whom the pacemaker is implanted, but substantially only on such physical activity and not on the vibrations and shocks which the host as a whole undergoes. It is also an object of this invention to provide a pacemaker for practicing this method.